Thermo-chemical surface treatments of steel by means of carbon or nitrogen carrying gases are well-known processes, called case-hardening or carburizing or nitriding. Nitro-carburizing is a process in which a gas carrying both carbon and nitrogen is used. These processes are traditionally applied to improve the hardness and wear resistance of iron and low alloyed steel articles. The steel article is exposed to a carbon and/or nitrogen carrying gas at an elevated temperature for a period of time, whereby the gas decomposes and carbon and/or nitrogen atoms diffuse through the steel surface into the steel material. The outermost material close to the surface is transformed into a layer with improved hardness, and the thickness of this layer depends on the treatment temperature, the treatment time and the composition of the gas mixture.
Stainless steel has excellent corrosion properties, but is relatively soft and has poor wear resistance, especially against adhesive wear. Therefore, there is a need of improving the surface properties for stainless steel. Gas carburizing, nitriding and nitro-carburizing of stainless steel involve some difficulties, as the passive layer, causing the good corrosion properties, acts as a barrier layer preventing carbon and/or nitrogen atoms from diffusing through the surface. Also the elevated temperatures of the treatments promote the formation of chromium carbides or chromium nitrides. Other alloys with a high chromium content, such as nickel base alloys, suffer from the same difficulties when it comes to case-hardening. The formation of chromium carbides and/or chromium nitrides reduces the free chromium content in the material, whereby the corrosion properties are deteriorated.
Stainless steel has iron as main constituent, whereas nickel base alloys have nickel as main constituent. Apart from chromium, a nickel base alloy may comprise cobalt, aluminium and other alloy elements.
Several methods of case-hardening stainless steel have been proposed by which the above mentioned drawbacks are minimized or reduced.
It is known that a pre-treatment in a halogen-containing atmosphere provides an effective activation of the surface.
EP0588458 discloses a method applying fluorine as an active component in a gas pre-treatment, where the passive layer of the stainless steel surface is transformed into a fluorine-containing surface layer, which is permeable for carbon and nitrogen atoms.
Plasma-assisted thermo-chemical treatment and ion implantation have also been proposed. In this case the passive layer of the stainless steel is removed by sputtering, which is an integrated part of the process.
EP 0248431B1 discloses a method for electroplating an austenitic stainless steel article with iron prior to gas nitriding. The nitrogen atoms can diffuse through the iron layer and into the austenitic stainless steel. After gas nitriding, the iron layer is removed, and a hardened surface is obtained. In the only example of this patent, the process is carried out at 575° C. for 2 hours. At this temperature, chromium nitrides are formed, whereby the corrosion properties are deteriorated.
EP 1095170 discloses a carburizing process in which an article of stainless steel is electroplated with an iron layer prior to carburizing. A passive layer is avoided, and carburizing can be carried out at a relatively low temperature without the formation of carbides.
WO 2004/007789 A1 discloses a process, wherein a layer of Ni, Ru, Co or Pd is applied to the surface of a stainless steel article prior to a case-hardening process, which is carried out below a temperature at which carbides or nitrides are formed. As disclosed in WO 2004/007789, chromium carbides are formed if carburizing is carried out above 550° C. Chromium nitrides are formed if nitriding is carried out above 450° C.
EP 818 555 A1 discloses a method for vacuum carburizing of steel by means of hydrocarbon gas. The process is carried out at temperatures up to 900° C.
Plasma and implantation based processes are a known method of treating an article However, plasma is not considered a method for gas carburizing an article, since it relies on the presence of ionized gas species, which are not present in gaseous treatment. Plasma processes have the disadvantage that accurate control of the carbon/nitrogen content is not possible on the basis of straightforward thermodynamics, but only empirically. In addition, only regions where a plasma can be generated or regions which are in the line-of-sight of the implantation gun can be treated. Moreover, the surface finish may suffer from extensive bombardment of ions (sputtering) during plasma/implantation treatment.
Alternatively, the use of a pre-treatment to activate the stainless steel surface prior to carbon/nitrogen introduction is known. Such pre-treatment involves removal of the natural oxide layer from the surface. The known pre-treatments use halogens, e.g. fluorine for the activation of the stainless steel surface which is associated with several drawbacks. One drawback is the fact that these types of gases are poisonous and highly aggressive and may furthermore be very detrimental for metallic parts in industrial furnaces. The gases can also initiate pitting corrosion in stainless steel impairing the “stainless” property of the steel. Also, exposure to aggressive gas (etching) may strongly deteriorate the surface finish of the stainless steel.